This novel process for cold working of copper alloyed materials having a relatively high lead content, of the order of at least 5%, has specific application in the fabrication of bearings and bearing bushings. While the specification and drawings are directed to and are specifically illustrative of this specific application, it will be understood that the cold working method of this invention can also be advantageously utilized in fabrication of other products or with other materials having similar characteristics.
Materials generally utilized for bearings or bearing bushings are copper alloys, generally termed bronzes. These alloys have copper as their major constituent but include significant amounts of other metallic elements. The most common alloys generally utilized to form bearings include significant amounts of lead, along with other metals such as tin, zinc and nickel, and are designated as leaded bronze. A distinct advantage of the copper-lead combination is that the lead imparts self-lubricative properties which enable bearings formed from such materials to operate in situations where external lubrication may not be continually assured. The primary disadvantages of the leaded bronze are that these alloys characteristically have tensile strengths and hardness properties which are inadequate for many bearing applications because of their limited load carrying capability. The most widely utilized leaded bronzes that are currently commercially available, with such an exemplary material being designated SAE 660 in the commercially accepted identification scheme have tensile strengths in the range of 25,000-45,000 P.S.I. and a hardness number in the range of 50-70 on the Brinell scale with a 500 kilogram load. These properties are obtained with the materials as cast which is the common condition for fabrication by the conventional machining operations since these materials have generally been considered unworkable by any conventionally known mechanical working techniques.
To accommodate more stringent bearing requirements and lead specifications for substantially greater tensile strength and hardness properties, copper alloys have been formulated with aluminum as the additional constituent of significant proportion. These copper-aluminum alloys do not include lead, and are not inherently self-lubricative. However, these copper-aluminum alloys do have substantially greater tensile strength and hardness thereby enabling bearings fabricated from these materials to withstand greatly increased loads. Such copper-aluminum alloys usually have tensile strengths of the order of 90,000 P.S.I. and a Brinell hardness of the order of 195 on the same Brinell scale as the leaded bronzes.
Enhancement of bronze materials properties has been proposed by addition of substantially significant quantities of tin. Exemplary of such proposals in U.S. Pat. No. 2,804,408 granted to Hardy E. Gregory on Aug. 7, 1957. Gregory discloses additional tin in proportions of up to 12% while the lead content remains low such as the order of 1%. Clearly, utilizing high proportions of tin provides enhancement of the alloy's structural strength characteristics but Gregory's disclosure is not remotely suggestive of using lead to enhance those properties with the disclosed processing technique.
As previously indicated, the leaded bronze type copper alloys are initially cast by various well known casting techniques. These techniques include the newer centrifugal and continuous casting as well as sand and permanent mold casting. The castings thus formed were then finished into a desired article, such as bearing bushing, by relatively costly machining operations in view of the generally accepted belief that these leaded bronze materials were mechanically unworkable by conventional rolling and forging procedures. These leaded bronzes cannot be hot worked because of the low melting point of the lead and other constituents that are included. Specifically, lead in the preferably significantly large proportions proposed by this invention liquefies at the temperature where the copper can be hot worked and lead, when liquefied, may pass out of or otherwise be lost from the material along with other low melting point constituents. Since hot working is prohibited by the low melting points of some of the lead and other constituents, it has generally been impractical to mechanically work the material utilizing normal rolling operations. Previous attempts at cold rolling have been restricted to relatively small dimensional reductions or changes in configuration as these materials tend to readily fracture when cold worked. Attempts at working of these materials beyond a predetermined limit generally results in fracturing of the material or separation of the various constituents. Although cold working increases the hardness of the material, this increase in hardness further increases the likelihood of the material to fracture.
In addition to the cost of the prior casting and machining operations for forming of bearing bushings, the bushings thus fabricated were limited to effectively have only the properties of tensile strength and hardness of the specific alloy as cast. As previously indicated, the materials available for fabrication of bearings are either of the leaded bronze or the aluminum bronze types and each type has its own respective strength and hardness properties. Consequently, selection of materials for bearings is limited to those having the customary formulations with the strength and hardness properties of the bearings fabricated therefrom being limited accordingly. It is not possible to alter the material compositions to the extent necessary to obtain different properties and, therefore, no bearing materials are reasonably available having strength and hardness properties intermediate the two widely separated ranges for the exemplary materials.